Scientists have discovered why the H. pylori bacterium, which infects half of the world's population, causes stomach problems in some people and not others.

Helicobacter pylori, which lives in the stomach, is thought to cause peptic ulcers and even gastric cancer.

But it was not known why only some people were affected in this way.

A Monash University team writing in Nature Immunology found some strains of the bacteria have a syringe-like device allowing them to affect stomach cells.

People have been wondering for a long time if there was something about some strains of the bacterium

Professor Charles Penn, University of Birmingham

Many people carry H. pylori without experiencing any symptoms.

And some of those who are affected suffer far less severe symptoms than others.

The researchers set out to discover how the bacterium behaved differently in different people.

They found that a component on the surface of the bacteria, called peptidoglycan, triggers stomach cells to undergo an inflammatory response.

Nod1, a molecule within the cells, recognises the peptidoglycan, and switches on signals that lead to inflammation developing.

Treatment aid

But the researchers were puzzled, because H. pylori does not normally invade cells.

However, they found that certain strains of H. pylori carry a syringe-like apparatus which "injects" the peptidoglycan into the cells.

The team say this is why only some people carrying H. pylori develop stomach disorders.

Charles Penn, professor of molecular microbiology at the University of Birmingham's School of Biosciences, told BBC News: "People have been wondering for a long time if there was something about some strains of the bacterium which made them behave differently.

"So this research supports that theory."

He said there had been other studies which suggested particular proteins from H. pylori were able to infect stomach cells.

Professor Penn added: "This latest work could be quite important because, if people know that someone is infected with the type of H. pylori which has this 'syringe' equipment, then it would be important to try and eliminate it."

H. pylori can be treated with powerful antibiotics such as amoxicillin or proton pump inhibitor drugs."